1. Field of the Invention
This invention relates to a WC-based cutting tool having a combination of toughness and deformation resistance which consists of a cemented carbide substrate with a stratified, cobalt-enriched surface zone and a double-layer coating of titanium carbide and titanium nitride. The term "stratified" refers to the layered appearance of the cobalt in the enriched zone. This substrate provides high thermal and mechanical shock resistance for maximum edge strength and increased insert toughness. By combining the enriched surface layer with the core substrate at 6% cobalt, the propagation of cracks from the coating is prevented and the cutting edge still maintains a minimum of deformation due to the heat and high forces generated at the cutting edge. The double layer of TiC and TiN, applied by chemical vapor deposition, enhances insert wear resistance and surface lubricity thereby enhancing tool life.
This cemented carbide cutting grade is ideal for heavy roughing applications (that is, high metal removal rates) on carbon and alloy steels, tool steels, stainless steels and cast iron. The inherent toughness of this grade, as a result of the cobalt distribution through the enriched zone, provides reliable performance in interrupted cuts and in heavy-scale or out-of-round conditions found in castings and forgings. The coating protects the substrate from abrasion and chemical attack from scale and by the steel being machined.
The present invention further relates to a process of making a cutting tool substrate, specifically to the achievement of a critical carbon level in the substrate, the slow cooling of the substrate to achieve a specific cobalt profile through the enriched zone, and maintaining high cobalt contents at the top of the enriched zones prior to coating, to make the tool suitable for heavy roughing applications.
The present invention also relates to the achievement of the critical carbon levels in a variety of cemented carbide compositions such that stratified enriched zones are formed during slow cooling, having the same cobalt profiles and hardness profiles as described above.
2. Description of Related Developments
Phase equilibria in the W--C--Co system have been reported by J. Gurland ["A Study of the Effect of Carbon Content on the Structure and Properties of Sintered WC--Co Alloys", Trans. AIME, 200, 285-290 (1954)] and by A. F. Guillermet ["Thermody namic Properties of the Co--W--C Sytem" Metall Trans A, 20A, 935-956 (1989)].
Lueth, U.S. Pat. No. 4,579,713 describes the adjustment of the carbon content of Co--WC compositions (straight grades only) using H2-CH4 gas mixtures in the temperature range 800.degree. to 1100.degree. C. It must be emphasized that these materials do not contain free carbon; that is, they are not in C-porosity. Also, the gas treatments are performed at well below sintering temperature while the parts are in their porous, un-sintered states.
In the technique disclosed by Lueth, CH4:H2 ratios are chosen so that the carbon activity is less than unity (easily calculated from the equilibrium reaction CH4.revreaction.C+2H2) so as to prevent the parts from moving into the free carbon region. The carbon activity of the parts will be controlled by the carbon activity of the gas phase. In this manner, initially high carbon parts can be decarburized, while initially low carbon parts can be carburized, and all will arrive at the same carbon level, or magnetic saturation value, within the two-phase WC+Co region of the phase diagram.
We have found this technique to be unsatisfactory for adjusting the carbon content of parts to specific levels within the free carbon region. It was far too sensitive to the CH4:H2 ratio, the temperature, the gas flow rates, and the manner in which the gases were introduced and removed from the furnace.
It has been known for several years within the cemented carbide industry that stratified enriched zones (so called because of the presence of wavelets, or strata, of cobalt parallel to the surface) can be produced by cooling parts containing free carbon. B. J. Nemeth, A. T. Santhanan and G. P. Grab ["The Microstructural Features and Cutting Performance of the High Edge Strength Kennametal Grade KC850", Proceedings of the 10th International Plansee Seminar, pp. 613-627 (1981)] were the first to describe this microstructure in a substrate having a nominal composition 6%Co-6%TaC-2%TiC-balance W and C. An appropriate amount of carbon was added to the powder batch in order to achieve the required carbon level. Details of the sintering and cooling of the parts (temperatures, cooling rates, gases used--if any, etc.) were not reported. Hardness measurements showed that the hardness increased continuously through the enriched zone and leveled off at a value appropriate for the interior.
Nemeth, et al., U.S. Pat. No. 4,610,931 have commented on the difficulty of controlling the carbon level, and hence the stratified enriched zones, in C-porosity substrates. In this patent, they described how a different type of cobalt enrichment, namely beta-free as opposed to stratified, can be obtained by the addition of hydrides, nitrides or carbonitrides of Group VB or VB transition elements to the powder mix.
This type of enriched zone is generated during vacuum sintering (due to the escape of nitrogen from the near-surface region, in the case of TiN or TiCN additions). These enriched zones are entirely free of solid solution carbide [W, Ti, Ta(Nb)C] grains, and they do not contain wavelets of cobalt. This type of enrichment occurs in cemented carbides having carbon levels ranging from eta phase to C-porosity, provided they contain any of the above additions (and, of course, solid solution carbides).
Taniguchi et al, U.S. Pat. No. 4,830,930 disclose the treatment of carburized transverse rupture strength (TRS) bars (composition 86%WC-5%TiC-7% Co) in a decarburizing atmosphere consisting of 10 torr H2-10% C02 mixture for 2 minutes at 1310.degree. C., followed by furnace cooling in a vacuum This resulted in a cobalt enriched surface layer in which the cobalt concentration actually went through a minimum before it approached the concentration of the bulk. There is no showing that the parts are in the C-porosity region.
Minoru et al, U.S. Pat. No. 4,911,989 disclose sintered and cooled parts pressed from powders containing excess carbon and titanium carbonitride powder, or treated parts containing excess carbon in nitrogen gas from 1000.degree. C. to 1450.degree. C. during the sintering cycle to achieve the same result. Such nitrogen-containing compositions gave rise to beta-free enriched zones, approximately 5 microns thick, which lie above the stratified enriched zones.
In one example, they have taken a composition not containing excess carbon or TiCN, sintered it at 1450.degree. C. and then cooled the furnace at 2.degree. C./minute down to 1310.degree. C. in an atmosphere of CH4 and H2, and then cooled the furnace to 1200.degree. C. at 0.5.degree. C./minute in a vacuum (10.sup.-5 torr) or in a C02 atmosphere. They claim the same hardness profile and the same 5 micron solid solution carbide-free layer at the surface.
Only one size of insert was studied (SNG 432, which is 1/2 inch square by 3/16 inch thick). The H2--CH4 gas composition and pressure are not reported, and the carbon level reached has not been reported. No showing is made as to the carburization of the parts.
We have not found it necessary to resort to such complicated cooling procedures in order to achieve hardness profiles in which the hardness increases continually through the enriched zone and levels off at the bulk value.
The as-sintered parts were then treated in acid to remove cobalt to depths of 2 to 5 microns below the top of the enriched zone. After coating the parts, the hardness at the original interface is greater than that of the bulk, and drops rapidly over this 2-5 micron region.
Okada et al, U.S. Pat. No. 5,106,674 have described the slow cooling in H2 and in CH4, over the temperature region 1380.degree. to 1300.degree. C. of compositions containing free carbon, to achieve a specific hardness profile through the enriched zones. The description is as follows: The hardness is essentially constant for a distance of approx. 10 microns from the surface of the part, and then increases to a value characteristic of the interior of the part. They state that these tools are suitable both for high speed cutting and for heavy duty cutting, as a consequence of the above-described cobalt distribution through the enriched zone.
Free carbon is present in the interior of their sintered parts, but they do not claim that a specific well-defined carbon level is required. The implication is that the carbon content of the part is not critical as long as it is somewhere in the C-porosity region prior to cooling, and that cooling the parts at rates of 0.2.degree. C./minute to 2.degree. C./minute in CH4 or H2 will result in the claimed cobalt distribution throughout the enriched zones.
No discussion is made as to the carburization of the parts. The emphasis in this patent is clearly on the cooling of the parts in CH4 or H2 in order to achieve a specific enriched zone description. Furthermore, during the cooling of the parts, if the CH4 or H2 pressure is outside of the limits 0.1 torr to 10 torr, or if the rate of cooling is outside the limits 0.2.degree. C./minute to 2.degree. C./minute, "it is impossible to obtain the aforementioned blade member according to the present invention".
Only one size of insert (style SNMG 120408, which is 12 mm square by 4 mm thick) is described in this patent.